Meteorite classification

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The ultimate goal of meteorite classification is to group all meteorite specimens that share a common origin on a single, identifiable parent body. This could be a planet, asteroid, Moon, or other current Solar System object, or one that existed some time in the past (e.g. a shattered asteroid).[citation needed] However, with a few exceptions, this goal is beyond the reach of current science, mostly because there is inadequate information about the nature of most Solar System bodies (especially asteroids and comets) to achieve such a classification.[citation needed] Instead, modern meteorite classification relies on placing specimens into "groups" in which all members share certain key physical, chemical, isotopic, and mineralogical properties consistent with a common origin on a single parent body, even if that body is unidentified. Several meteorite groups classified this way may come from a single, heterogeneous parent body or a single group may contain members that came from a variety of very similar but distinct parent bodies. As such information comes to light, the classification system will most likely evolve.

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Beyond the assignment of meteorites into groups (see above), which is essentially universally accepted, there is no consensus among researchers as to what hierarchy of classification terms is most appropriate. For chondrites, groups may be divided into subgroups where there are features that distinguish certain meteorites from the others in the group, but it is thought that all still come from a single parent body. It is also fairly common for groups that seem to be closely related to each other to be referred to as clans. In turn, groups or clans that appear to be loosely related are often referred to as chondrite classes (e.g., carbonaceous chondrites, enstatite chondrites, and ordinary chondrites). But higher order terms for aggregating groups of meteorites tend to be somewhat chaotic in the scientific and popular literature. There is little agreement on how to fit nonchondritic meteorite groups into an overall scheme.

Ungrouped, meteorites that do not fit any known group, though they may fit into a clan or class[citation needed] (e.g., the meteorite Acfer 094 is in an ungrouped member of the CM-CO clan of carbonaceous chondrites).

Meteorites are often divided into three overall categories based on whether they are dominantly composed of rocky material (stony meteorites), metallic material (iron meteorites), or mixtures (stony–iron meteorites). These categories have been in use since at least the early 19th century but do not have much genetic significance; they are simply a traditional and convenient way of grouping specimens. In fact, the term "stony iron" is a misnomer as currently used. One group of chondrites (CB) has over 50% metal by volume and contains meteorites that were called stony irons until their affinities with chondrites were recognized. Some iron meteorites also contain many silicateinclusions but are rarely described as stony irons.[citation needed]

Nevertheless, these three categories sit at the top of the most widely used meteorite classification system.[citation needed] Stony meteorites are then traditionally divided into two other categories: chondrites (groups of meteorites that have undergone little change since their parent bodies originally formed and are characterized by the presence of chondrules), and achondrites (groups of meteorites that have a complex origin involving asteroidal or planetary differentiation). The iron meteorites were traditionally divided into objects with similar internal structures (octahedrites, hexahedrites, and ataxites), but these terms are now used for purely descriptive purposes and have given way to modern chemical groups. Stony–iron meteorites have always been divided into pallasites (which are now known to comprise several distinct groups) and mesosiderites (a textural term that is also synonymous with the name of a modern group).

Below is a representation of how the meteorite groups fit into the more traditional classification hierarchy:{[1] }

Two alternative general classification schemes were recently published by Krot et al. (2003)[4] and Weisberg et al. (2006),[5] illustrating the lack of consensus on how to classify meteorites beyond the level of groups.[citation needed] In the Krot et al. scheme, the following hierarchy is used:

(Sh) Shalkite: Granular. Olivine and bronzite. [[Tschermak had included this category from Rose (1863), but he discovered in 1883 that the Shalka meteorite did not contain any olivine and proposed the term "diogenite" for hypersthene achondrites.[9]]]

He subdivides the "Meteoric Stones" into those that have chondrules (Chondritic Meteoric Stones or Chondrites) and those that don't (Non-chondritic Meteoric Stones or Achondrites). The iron meteorites are subdivided according to their structures as ataxites, hexahedrites and octahedrites.[15] A complete overview of his classification is given in the box below: